1. Field of the Invention
The present invention relates to a structure and manufacturing method of a chip type resistor having a resistive film and terminal electrodes positioned at opposing ends of the resistive film, formed on a chip type insulating substrate.
2. Description of the Background Art
In a conventional chip type resistor such as described in Japanese Patent Laying-Open No. 60-27104, a cover coat covering a resistive film formed on a surface of a chip type insulating substrate is protruded from a surface of terminal electrodes positioned at opposing ends of the resistive film, so that there is a considerable step between the surface of the cover coat and the surface of the terminal electrodes. Therefore, it suffers from the problem that when the chip type resistor is soldered on a printed board with the side of the resistive film facing the printed board, one side of the chip type resistor rises or floats, preventing secure soldering.
In view of the foregoing, Japanese Patent Laying-Open No. 4-102302 discloses a structure of a chip type resistor in which terminal electrodes 3 are formed at opposing ends of a resistive film 2 on left and right end portions of a chip type insulating substrate 1 such that the terminal electrodes 3 each includes a main upper electrode 3a formed on the surface of insulating substrate 1 and conductive to resistive film 2, an auxiliary upper electrode 3b formed heaped up on the surface of main upper electrode 3a, a side electrode 3c formed on either side of insulating substrate 1, and a metal plate layer 3d formed over the surfaces of auxiliary upper electrode 3b and side electrode 3c, as shown in FIGS. 123 and 124. This laid-open patent application proposes, by this structure, to reduce or eliminate the step between the surface of terminal electrode 3 and the surface of cover coat 4 covering resistive film 2.
Cover coat 4 has a three-layered structure including an undercoat 4a directly covering resistive film 2, a middle coat 4b covering undercoat 4a, and an overcoat covering middle coat 4b. The undercoat and/or middle coat may be omitted.
The chip type resistor disclosed in Japanese Patent Laying-Open No. 4-102302 is manufactured through the following steps.
Step 1. On an upper surface of the insulating substrate, main upper electrodes 3a are formed and thereafter resistive film 2 is formed. Alternatively, resistive film 2 is formed first and thereafter main upper electrodes 3a are formed.
Step 2. Undercoat 4a of glass is formed on resistive film 2 (the undercoat may be omitted). Thereafter, resistive film 2 and undercoat 4a are engraved to form a trimming groove by laser beam irradiation, for example, while resistance value of resistive film 2 is measured by a conductive probe which is brought into contact with main upper electrodes 3a so that the resistance value of resistive film 2 is within a prescribed tolerable range.
Step 3. Middle coat 4b of glass is formed on the surface of undercoat 4a to fill the trimming groove. (The middle coat may be omitted.) Thereafter, overcoat 4c of glass or a synthetic resin is formed covering resistive film 2, part of the main upper electrodes 3a, undercoat 4a and middle coat 4b.
Step 4. On the surface of main upper electrodes 3a, auxiliary upper electrodes 3b are formed by heaping up a conductive paste. Thereafter, side electrodes 3c are formed on end surfaces of insulating substrate 1, and surfaces of auxiliary upper electrodes 3b and side electrodes 3c are subjected to metal plating, whereby a metal plate layer 3d is formed.
In the above described chip type resistor, the step between terminal electrodes 3 and cover coat 4 is reduced or eliminated by forming auxiliary electrodes 3b on the upper surfaces of the main upper electrodes 3a by heaping up the conductive paste thick. In other words, auxiliary upper electrodes 3b are formed simply by heaping up the conductive paste. Therefore, in order to reduce or eliminate the step between terminal electrodes 3 and cover coat 4, considerably large amount of conductive paste is used, which leads to higher cost for the material and therefore higher manufacturing cost, as well as to increased weight of the chip type resistor.